Sensing a mass deposited onto a surface of a piezoelectric resonator is a technique that artisans in the measuring and testing field have used for decades. A conventional quartz crystal microbalance (QCM) typically includes a piezoelectric resonator capable of sensing loads less than a microgram. For small amounts of mass, a change in a resonant frequency of a piezoelectric resonator is proportional to a mass change. Thus, QCM's operate in a variety of diverse applications. For example, QCM's often operate as detectors for measuring humidity or the presence of other adsorbed gases in an atmosphere. In addition, QCM's operate as sensors for monitoring film thickness in thin-film deposition processes.
In the past, fabricators generally designed QCM sensors to operate in air or other gaseous environments. More recently, QCM sensors operate in liquids. The following article describes a specific application of an acoustic sensor having a quartz crystal resonator that operates in oil: Hammond et al., "AN ACOUSTIC AUTOMOTIVE ENGINE OIL QUALITY SENSOR," Proceedings of the 1997 IEEE International Frequency Control Symposium, IEEE Catalog No. 97CH36016, pp. 72-80, 28-30, May 1997.
The Hammond et al. article notes that the viscosity of oil in an automobile engine is perhaps the single most important technical parameter of a modern crankcase lubricant. Thus, Hammond et al. propose an onboard sensor for measuring viscosity changes of crankcase oil in an automobile or other similar mechanism. They describe a technique of measuring the viscosity of oil by operating an AT-cut quartz resonator immersed in the oil. The sensor includes a drive circuit that excites a shear mechanical motion in the resonator, which motion transfers to the oil as a shear wave. The oil essentially acts as a mechanical load to the quartz resonator and this mechanical load affects the quality factor Q and other electrical properties of the resonator. The Hammond et al. article describes how a change in the electromechanical quality factor Q of a resonator is proportional to the mass accumulation at the resonator-oil interface. In addition, the article explains that changes in the resonant frequency and the amplitude of a resonance signal due to the mechanical loading are each proportional to the square-root of a product of the density and viscosity of a liquid. Thus, Hammond et al. measure the combined effects of phase and amplitude changes of a sensing signal to monitor changes in an oil viscosity.
Others have used similar techniques to measure the properties of a variety of different liquids. The following articles describe resonator sensors capable of making simultaneous measurements of liquid density and viscosity: Zhang et al., "CONTRIBUTIONS OF AMPLITUDE MEASUREMENT IN QCM SENSORS," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 43, No. 5, pp. 942-947, September 1996; and Martin et al., "MEASURING LIQUID PROPERTIES WITH SMOOTH- AND TEXTURED-SURFACE RESONATORS," 1993 IEEE International Frequency Control Symposium, IEEE Catalog No. 93CH3244-1, pp. 603-608, June 1993.
The Zhang et al. article describes how a QCM, having an AT-cut quartz resonator, detects changes in viscosity and density of a liquid. This article indicates that when a QCM operates in a liquid, the total frequency change consists of two effects, one due to mass loading and the other due to "liquid damping." Further, according to Zhang et al., one cannot distinguish a mass loading effect from a total frequency change by only frequency measurement. Thus, a standard technique of using a QCM in liquids is to simultaneously measure changes in a frequency and a quality factor Q (or changes in equivalent circuit parameters). This allows separation of mass loading effects from liquid damping effects.
The Martin et al. article describes an improved method that uses a dual-resonator sensor with two AT-cut quartz resonators, one with a smooth surface and the other with a textured or rough surface. The surface texture comprises ridges oriented perpendicular to the direction of a surface shear displacement, i.e., the X crystalline direction. When operated in a liquid, the smooth resonator generates plane-parallel laminar flow in an adjacent liquid, which causes a resonator frequency shift that is a function of liquid density and viscosity. A textured resonator, however, traps a quantity of liquid in excess of that entrained by a smooth surface. The trapped liquid behaves as an ideal mass layer, causing an additional frequency shift that depends only on density and not viscosity.
In the Martin et al. sensor, each resonator is driven by an independent oscillator circuit that provides the following two outputs: a radio frequency (RF) signal that tracks resonant frequency and a direct current (DC) voltage proportional to motional resistance. Baseline responses are determined by measuring resonant frequency and motional resistance for each resonator before its immersion in a liquid. Changes in resonator responses are then measured separately for the smooth and textured resonators after immersion. A computer connected to the sensor calculates density and viscosity. In particular, the liquid density is first calculated from the difference in responses measured between the smooth and textured devices. Having determined liquid density, the response of the smooth resonator is then used to calculate liquid viscosity. Thus, the Martin et al. method measures a frequency change and a change in quality factor Q (or a change in equivalent circuit parameters) for each resonator separately.
Although standard techniques of sensing the properties of fluids have served the purpose, they have not proved entirely satisfactory when making highly sensitive measurements of fluid properties, including viscosity and density. Sensor designers acknowledge that while changes in frequency are usually measured with great accuracy, changes in quality factor Q, motional resistance or any other quantities are measured with significantly less accuracy. Q measurements for high-Q devices are typically made with an accuracy of two to four significant figures, whereas the frequencies of stable frequency sources can be measured with an accuracy of 14 significant figures. For low-Q devices, such as resonators immersed in a fluid, the accuracy of the Q and frequency measurements is lower; however, the accuracy of the frequency measurements is still orders of magnitude higher than the accuracy of the Q measurements.
Sensor fabricators have also recognized problems with using resonators with smooth and textured surfaces. Changes in frequency and Q depend not only on a liquid's properties, but also on a resonator's surface roughness. However, it is difficult to produce surfaces of identical surface roughness, i.e., it is difficult to produce a "standard" rough surface.
An additional difficulty with the prior art is that temperature can greatly affect the properties of fluids, such as a fluid's viscosity. It is well known that, for example, the viscosity of many oils and lubricants vary with temperature and degradations due to chemical changes. Measuring frequency and Q changes alone cannot determine the temperature of a fluid simultaneously with the fluid's viscosity and density. Therefore, when only frequency and Q are measured, and a viscosity change is detected, it is not possible to determine the cause of the viscosity change. The change could be due to a temperature change or to a change in the quality of the fluid, or to a combination of such factors.